Response Of Community Structure And Function To Environmental Disturbance In The Alpine Meadow Of Tibetan Plateau

The effects of environmental disturbance on plant communities, especially for the alpine meadow with tough habitat and fragile ecosystem stability, has been an issue in ecology. Exploring the response of community structure and function to different environmental disturbance(e.g. grazing and fertilization) is helpful to provide theoretical basis for the management and recovery of degraded alpine meadow. We carried out a series of field observation and artificial control experiment in the alpine meadow to probe the response of plant community structure and function to different environmental disturbance. The main objectives are as follows:(1) To explore the response of community structure and function to long-term fertilization.(2) To explore the recovery pattern of community structure and function after the cessation of fertilization.(3) Which environmental factor caused species diversity loss due to fertilization, light resource limitation or soil nutrients alteration?(4) To explore the maintenance mechanism of species diversity with the fertilization and grazing. The main results are as follows:1. The effects of long-term fertilization on community structure and function in alpine meadow.Long-term fertilization significantly increase community productivity and decrease plant species diversity, and the alteration trends become more apparent along the gradients of fertilization and temporal scale. The composition of plant community transform from sedge species and forb species to simple gramineous species along temporal scale; the community structure and function could reach a new equilibrium state with enough time. After fertilization, sedge species and legume species were lost quickly from community, and the gramineous species become unique dominant species in the fertilized community. In addition, fertilization decrease soil p H and increase soil available nitrogen, both of them are significantly correlate with species diversity; the relative abundance of species is also significant correlate with mean individual height. The results suggested that soil nutrients and light resource may jointly determine the species loss after fertilization.2. The recovery patterns of community structure and function in response to the cessation of fertilizationAfter the cessation of fertilization, species diversity in all the fertilized community show significantly increase trend. However, the four year is not enough for the recovery of community composition. The recovery of legumes species is more quickly than sedges species. The aboveground biomass of fertilized community recover to the level of the control plots after the cessation of fertilization. Soil p H, soil available nitrogen and individual height are significantly correlated with spcies diversity. The results stated that the loss and recovery of plant species related to fertilization disturbance may be reversible and the patterns are determined by the soil nutrients and species functional traits.3. The role of light competition hypothesis in the process of species loss due to fertilizationBoth of fertilization and shade can cause species loss in the alpine meadow of Tibetan plateau, which is a robust evidence to support light completion hypothesis. Fertilization increase aboveground biomass of the community and then decrease photosynthetically active radiation(PAR) in the understory; soil nutrients are not alterd by shadecloth, and the species diversity is not correlated with aboveground biomass. The lost species with the two treatments are not identical. The lost species identity are correlated with species individual height in fertilized plots, and the lost species identity are correlated with their light compensation point in shaded plots.4. The maintenance mechanism of species diversity with fertilization and grazing treatment.Fertilization and grazing influence species diversity via multiple pathways. The overall effect of fertilization on species diversity are negative. Specifically, the direct effect of fertilization on species diversity are negative, mainly attribute to the alteration of soil physical and chemical properties; the indirect effect of fertilization on species diversity are negative, mainly mediated by increasing aboveground biomass. The overall effect of grazing on species diversity are neutral effect. Specifically, the direct effect of grazing on species diversity are negative, mainly attribute to the foraging behavior; the indirect effect of grazing on species are positive, mainly mediated by decreasing aboveground biomass and increasing belowground biomass. The negative effect of fertilization on plant species diversity are masked by grazing, though no significant interaction are found.